The present invention relates to a zoom lens system and, more particularly, to a compact standard zoom lens system for use with such cameras as a single-lens reflex camera and a video camera.
Among interchangeable lenses for 35-mm still cameras, there are zoom lens systems which comprise two lens units, that is, a first lens unit having negative refractive power, and a second lens unit having positive refractive power, which are disposed in order from the object side. One type of such two-unit zoom lens system covers a standard angle of view and provides a zoom ratio of about 2 (this type of zoom lens system will hereinafter be referred to as "standard zoom lens system"). Recently, the use of such a standard zoom lens system in place of a standard lens system (i.e., a lens system which has a focal length of about 50 mm in the case of 35-mm camera) has become common in the art.
Accordingly, such a standard zoom lens system is attached to the camera body as a lens system for ordinary use, and it is therefore essential to minimize the size of the zoom lens system. It is also necessary to produce the standard zoom lens system in a compact form and at low cost while ensuring satisfactory image-forming performance.
The most suitable lens type for realizing such a standard zoom lens system is the above-described two-unit zoom lens system, which comprises a first lens unit having negative refractive power, and a second lens unit having positive refractive power. In regard to this type of zoom lens system, various lens arrangements have heretofore been proposed.
For example, Japanese Patent Laid-Open Publication (KOKAI) No. 56-123512 proposes a zoom lens system comprising only spherical lens elements.
Japanese Patent Laid-Open Publication (KOKAI) Nos. 01-239516 and 05-88084, filed by the present applicant, propose a zoom lens system which has realized high performance and a compact structure by introducing an aspherical surface.
Meanwhile, a two-unit zoom lens system comprising 7 lens elements has been proposed in, for example, Japanese Patent Laid-Open Publication (KOKAI) Nos. 01-185607 and 56-158315. Japanese Patent Laid-Open Publication (KOKAI) No. 61-80214 proposes a two-unit zoom lens system which has achieved a compact structure by using from 7 to 8 lens elements.
These conventional zoom lens systems suffer, however, from the following problems:
The zoom lens system disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 56-123512 comprises a relatively small number of lens elements, all of which are spherical lens elements. Therefore, it is favorable from the viewpoint of cost. However, the overall length of the lens system (hereinafter referred to as simply "overall length") at the wide end of the zoom range is considerably long. Thus, the proposed zoom lens system cannot be said to be compact as a whole. The zoom lens system also involves many problems in terms of aberration correction; it is not satisfactorily corrected for spherical aberration, particularly at the tele end of the zoom range. Accordingly, the zoom lens system needs further improvement as a two-unit zoom lens system having a zoom ratio of about 1.9.
The zoom lens system that is disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 01-239516 has realized a high performance and a compact structure by introducing an aspherical surface. However, an optical system that includes an aspherical surface tends to be costly in comparison to an optical system comprising only spherical lens elements. In addition, since aspherical surface processing is more difficult than spherical surface processing that has been established by conventional technique, an optical system including an aspherical surface is disadvantageous when it is intended to reduce the cost to the utmost limit.
It should be noted that in the zoom lens system is disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 01-239516, first and second lens units, which are disposed in order from the object side, have a negative meniscus configuration. This lens system is effective for further widening the viewing angle, but, on the other hand, it suffers from the disadvantage that the diameter of the front lens becomes relatively large because the height of obliquely incident light rays is relatively high. It should be noted that, in a case where the viewing angle at the wide end is of the order of 2.omega.=64.degree. as in the present invention, it is preferable for the second lens unit to have a planoconcave or biconcave configuration with a view to reducing the filter diameter and also reducing the size of the first and second lens units to thereby achieve a reduction in the overall weight and the overall cost.
The zoom lens system disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 05-88084 is an example of a lens arrangement which comprises the smallest number of lens elements among the zoom lens systems of this type, and hence excellent in terms of the achievement of a compact structure and a reduction of the cost. However, this zoom lens system involves a problem in terms of aberration correction. That is, the zoom lens system suffers from a somewhat large field curvature at the tele end.
Meanwhile, the zoom lens system that is disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 01-185607 realizes a two-unit zoom lens system comprising 7 lens elements without using an aspherical surface or the like. In this two-unit zoom lens system, however, the zoom ratio is disadvantageously small, and the overall length is unfavorably large. Thus, the reduction in size of the lens system has not satisfactorily been achieved.
In addition, each lens element that constitutes the first lens unit of the above-described zoom lens system is large in size. In this regard also, the zoom lens system needs to be improved in order to achieve a compact structure and a reduction in cost. From the viewpoint of performance also, the zoom lens system suffers from the problem that the variation of spherical aberration remains at the tele end in particular, and the variation of the lower comatic aberration caused by zooming also remains.
Further, the zoom lens system that is disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 56-158315 also suffers from the problems that the overall size is relatively large, and that the zoom ratio is disadvantageously small.
In the above-described zoom lens system, a negative lens element in the first lens unit which is disposed at the second position from the object side has a planoconcave or negative meniscus configuration. Accordingly, the diameter of the front lens becomes relatively large contrary to the intention of achieving a compact structure, resulting in an increase in cost.
Furthermore, the zoom lens system that is disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 61-80214 provides a relatively small zoom ratio and has a relatively small air spacing between the negative and positive lens elements among three lens elements, i.e., negative, negative and positive lens elements, which constitute the first lens unit. Accordingly, field curvature, astigmatism and negative comatic aberration cannot satisfactorily been corrected.
In general, when a two-unit zoom lens system, which comprises a negative lens unit and a positive lens unit, satisfies the relationship of the following expression (a), the overall length at the wide end and the overall length at the tele end are equal to each other, and the change in the overall length of the lens system caused by zooming is the smallest. EQU f1=-(fw.multidot.ft).sup.1/2 (a)
where "fw" is the focal length of the entire lens system at the wide end, "ft" is the focal length of the entire lens system at the tele end, and "f1" is the focal length of the first lens unit G1.
Accordingly, it is not preferable to select the focal length f1 of the first lens unit G1 so that the zoom lens system deviates from the relationship of the expression (a) to a considerable extent, because the change in the overall length of the lens system caused by zooming would become excessively large.
Assuming that the magnification at the tele end of the second lens unit G2, which is a convergent lens unit, is .beta.t, the relationship of the following expression (b) holds: EQU ft=f1.multidot..beta.t (b)
It is necessary in order to reduce the size of the zoom lens system to use the second lens unit G2 such that the magnification thereof at the tele end exceeds 1. In order to satisfy the relationships of the above expressions (a) and (b) while making favorable aberration correction with a compact structure, it is necessary to use each lens unit having a relatively strong refractive power. Accordingly, in the conventional zoom lens systems, there has been a tendency to compose each lens unit of a relatively large number of lens elements. As a result, each lens unit has heretofore been likely to increase in thickness, thus weakening the effect of reducing the overall size to achieve a compact structure.
In view of the above-described problems of the related art, an object of the present invention is to provide a zoom lens system which has a high zoom ratio, a compact structure, a simple arrangement, a reduced cost, and yet favorable image-forming performance.
To attain the object, the present invention provides a zoom lens system wherein a refractive power distribution suitable for achieving a reduction in size of the lens system is set by taking into consideration the above-described two expressions (a) and (b), and wherein a first lens unit G1, which is a divergent lens unit, comprises 3 lens elements, that is, a negative lens element L1, a negative lens element L2, and a positive lens element L3, thereby realizing a zoom lens system having a compact structure, a minimal change in the overall length caused by zooming, and a reduced cost.